Electrical coupling system



April 18, 1939. MQUNTJOY 2,154,692

ELECTRICAL COUPLING SYSTEM Filed Jan. 21, 1957 30 2 p q Q 6 7 a 9 /o /2 a /4 l5xf0 FREQUENCY ATTORNEY Patented Apr. 18, 1939 UNITED sures 2,154,692 ELECTRICAL COUPLING SYSTEM Garrard Mountjoy,

as'sign'or to Radio Bayside, Long Island, N. Y., Corporation of America, a

corporation of Delaware Application January 21, 1937, Serial No. 121,424

7 Claims.

An object of this invention is to provide a coupling system for connecting an antenna, and more especially a capacity type of antenna, to the input terminals of a radio receiver.

A more specific object is to provide an efficient coupling system wherein the capacity of the antenna is of small value such as in the'case of a rod antenna when mounted on a vehicle such as an automobile, in which case the coupling system may also be designed to prevent extraneous noise currents, such as those originating inthe ignition circuits of the engine, from gettinginto the receiver.

A still furtherobject is to provide a' coupling system for an antenna which substantially increases thegain at the lower'frequenciesof the tuning range andthereby provides a more uniform gain over the entiretuningrang'ethan the usual type of coupling system commonly used.

Further objects of the invention will become apparent to those skilled in the art as the description thereof proceeds. For a better understanding of the invention, however, reference is made to the accompanying drawing in which;

Fig. 1 is a schematic circuit diagram of a coupling system embodying the invention and,

Fig. 2 is .a curve showing the gain obtained with a coupling system of the type shown in Fi 1.

As shown in Figure 1, a rod antenna I composed of a short metal rod or other suitable type of capacity antenna is connected to a coil L3 by a short lead 14 and through a coil L2 to ground or a counterpoise II. It will be understood that my invention is not limited to the use of a rod antenna as any known type of antenna may be used therein and more especially an antenna whose capacity is relatively small. As shown, the antenna capacity with reference to ground is indicated by the dotted condenser of capacity Ca. The lower end of coil L3 is connected by the leadin wire 12, which is preferably enclosed within a flexible metallic shield indicated at I3, to one side of a condenser Cc which provides capacity coupling between the antenna circuit and the resonant circuit including coil L1 and its shunting condenser CT. Condenser CT is of the known adjustable type employing rotor and stator portions and is used in conjunction with coil L1 to form a parallel tuned circuit adjustable to resonate at the desired signal frequency. As shown, the output terminals I5 of the coupling system are connected to the control grid and cathode of the radio frequency amplifying stage of a radio receiver, the other stages of which are not shown andjmay be of the tuned radio frequency type or of the superheterodyne type such as known to those skilled in the art. .3;

The capacity between the lead-in l2 and 6 ground, which includes the capacity between it and its enclosing shield !3 which is preferably grounded, is indicated by the dotted capacity CL. I prefer to mount the the antenna and coil L2 in the same casing with 0 the radio receiver so that these coils are not inductively coupled although a small degree of inductive coupling between them would not materially alter the performance of the system. When used to receive modulated carrier signals in the present broadcast band of 550 to 1500 kilocycles, the circuit constants are designed so that L3 and Ca resonate at 1500 kilocycles or at a frequency'slightly above this value and also Ca, L3, Lzand CL form a circuit resonant at 600 kilocycles. Defining the gain as the ratio of the voltage developed across output terminals |5 to the voltage induced in the antenna by the received carrier wave, a coupling system designed as above gave the gains shown in the curve of Fig. 2 as the tuning condenser CT was adjusted for the several frequencies of the band. It will be noted that the gain does not fall off at the lower frequencies to as great an extent as when simple transformer coupling between the antenna and the resonant input circuit of the receiver is employed.

In this particular installation the circuit elements had the following values:

Ca=11 micro-microfarads CL=30 micro-microfarads Cc=20 micro-microfarads L1=250 microhenrys L2=1800 microhenrys La=750 microhenrys In an automobile installation the disturbing high frequencies radiated by the ignition system or other wiring circuits of the car and which may reach the antenna I may be prevented from reaching the receiver by designing coil L3 to offer a high impedance to currents of such frequencies, which are usually higher than the broadcast band. The coil should be preferably wound so that its windings or an appreciable part thereof, have a low distributed capacity. I also prefer to make the coupling condenser Cc adjustable so that it may be set and function in the fashion of a trimmer condenser to adjust the resonant frequency of the circuit L1 CT to a definite value at the high frequency end of the tuning range or coil L3 closely adjacent to when tuning condenser CT is set at its position of minimum capacity.

While I have described my invention in connection with a circuit tunable over the broadcast band of frequencies, it will be understood that it is equally adapted for use to connect an antenna to a circuit tunable over either a higher or a lower range of frequencies.

While I have described my invention in connection with a certain circuit, it will be understood that it is not limited thereto but includes all transmission circuits included within the scope of the following claims.

What I claim and desire to secure by Letters Patent is:

1. In combination, a source of signal voltage having a predetermined capacity, a parallel resonant circuit tunable over a wide range of frequencies, a coil of low distributed capacity having one end connected to said voltage source and having such an inductance value as to resonate with the capacity of said voltage source at a frequency of the order of the highest frequency of said range of frequencies and a condenser connected to the other end of said coil and to one end of said parallel resonant circuit.

2. In combination, an antenna having a substantially fixed capacity, a parallel resonant circuit tunable over a wide band of signal frequencies, a coil and condenser connected in series between said antenna and the high potential end of said resonant circuit, said coil having low distributed capacity and forming with the antenna capacity a circuit having a resonant frequency at substantially the highest frequency of the band.

3. In combination, a source of signal voltage having a predetermined capacity, first and second coils connected in series between said voltage source and ground, the first of said coils being of low distributed capacity and forming with the :inherent capacity of said voltage source a circuit having a resonant frequency of the order of w, a parallel resonant circuit tunable over a frequency range from w to a substantially lower frequency and a condenser of small capacity having one side connected to the common terminal of said coils and its other side connected to the high potential end of said parallel resonant circuit.

4. The arrangement of claim 3 in which said coils are separated to such an extent that there is substantially no inductive coupling therebetween.

5. Thearrangement of claim 3 in which said first coil is located in close relation to the signal source and is constructed to have an inductance suificiently high as to substantially prevent the transmission of currents of frequencies much higher than (0.

6. In a transmission network, the combination of a source of signal voltage having a predetermined capacity, first and second coils connected in series between said source and ground, a parallel resonant circuit tunable through a band of frequencies, a condenser having one side connected to the high potential end of said parallel resonant circuit, a lead-in connected between the other side of said condenser and the common terminal of said coils, the inductances of said coils beingso designed that together with the inherent capacity of said voltage source and said lead-in they form a circuit having a resonant frequency of the order of the lowest frequency of the band.

'7. The arrangement of claim 6 in which the first coil is of low distributed capacity and has such an inductance as to form with the inherent capacity of said voltage source a circuit having a resonant frequency of the order of the highest frequency of the band.

GARRARD MOUNT-JOY. 

